The present invention relates to remote sensing of materials and more particularly to a remote sensor for fissile or nuclear material. Even more particularly, the present invention relates to a remote sensor for fissile material utilizing optical filters for filtering out light outside a pre-determined radiation spectral band selected according to certain naturally occurring properties of the selected band up to a certain earth altitude, with a relatively high mean free path in air, and a sufficiently short radiative emission rate with photons up to the earth altitude.
Proliferation of weapons of mass destruction has generated a need to detect and locate fissile material that may be fabricated into nuclear weapons and to detect nuclear weapons themselves (hereinafter collectively referred to as nuclear materials). Unfortunately, these nuclear materials are difficult to detect with available nuclear material detectors (such as gamma ray detectors) because these detectors, in practice, must be held a few tens of meters or less from a nuclear source in order for detection to occur.
It would greatly improve the effectiveness of a nuclear materials detector if the detector could be mounted on an aircraft or flown on a satellite and could reliably and remotely detect fissile material from distances on the order of kilometers, rather than meters. Advantageously, this would allow detection of nuclear materials without the need to have an inspector on site to carry out the inspection.
The inventors are not aware of any heretofore available and practical technologies that allow remote detection of fissile material, such as at distances on the order of kilometers.
The current state of the art in nuclear material detection (i.e., detection of fissile material) has been summarized in a Los Alamos report entitled xe2x80x9cFinal Report: Scoping Study of SNM Detection and Identification for Adjunct On-site Treaty Monitoring.xe2x80x9d
Nuclear material detectors are currently categorized as three types of detectors, Gas filled, scintillation or solid state detectors. Gas filled detectors have a sensitive volume of gas contained within a sealed chamber between two electrodes. The chamber allows ionizing radiation from outside the chamber to enter the chamber, and may be, for example, glass. There are three types of gas filled detectors. They may be: 1) an ionization chamber; 2) a proportional counter; or a 3) Geiger-Mueller tube (GMT).
In each of these three types of gas filled detectors, the electrodes are biased with a biased power supply. An ionization event within the gas is caused by the ionizing radiation entering the gas. This causes the generation of electron hole pairs that are, in turn, collected by the two electrodes.
In an ionization chamber only primary charge created from a first ionizing event with the ionizing radiation are collected due to a low voltage in the ionization chamber. As voltage on the electrodes is increased due to the collection of electron hole pairs, the primary charge attains enough energy to ionize additional molecules. (This creates a mechanism called avalanche amplification, also used in both xe2x80x9cproportional countersxe2x80x9d and Geiger-Mueller tubes.)
In proportional counters, the avalanche amplification that occurs when the primary charge attains enough energy to ionize additional molecules is used to generate a xe2x80x9ccountxe2x80x9d. The number of xe2x80x9ccountsxe2x80x9d generated over time provides an indication of the amount of ionizing radiation present and thus an indication as to whether, and how much, nuclear material is present.
Both ionization chambers and proportional counters collect charge generated as a result of ionization events, with the amount of charge over time being proportional to energy deposited in the gas as a result of ionization events. Both measure ionizing radiation by measuring collected charge from electron hole pairs collected by the electrodes, represented by voltage. However, because of their efficiency level they are limited to detecting x-rays rather than gamma rays generated by fissile materials. And further, because they directly detect ionizing radiation (through ionization events) they must be used within close proximity of the nuclear materials. (This is because ionizing radiation is consumed naturally by ionization events as the ionizing radiation travels through space, particularly through an atmosphere, such as at the surface of the Earth.) Thus, the amount of measured ionizing radiation quickly fades into background radiation levels varying as a function of distance (on the order of meters generally) from the particular nuclear materials from which the ionizing radiation is being emitted.
If the electrode voltage on a xe2x80x9cproportional counterxe2x80x9d detector is increased further, the ionization within the gas becomes space-charge limited and the charge produced is independent of the initial deposition of energy in the gas. This type of detector is termed a Geiger-Mueller tube (GMT) and cannot differentiate between the energy level of the particle it detects. Thus, in addition to being unable to detect nuclear material at larger distances, Geiger-Mueller tubes are unable to differentiate between different types of radiation sources.
A further type of nuclear material detector, a scintillation detector, uses scintillation, which occurs when ionizing radiation strikes a luminescent material (xe2x80x9cscintillator materialxe2x80x9d) such as, NaI, BGO, CsI, ZnS or LiI. A scintillation detector is a device that detects gamma ray induced scintillation emissions (xe2x80x9cscintillatorsxe2x80x9d). For example, gamma rays cause scintillations by exciting atoms that emit optical photons (light) as the atoms decay back to a ground state. Optical photons have energies corresponding to 2000-15,000 A.
In a scintillation detector, isotropically emitted photons are optically coupled to a photocathode of a Photo-Multiplier Tube (PMT), which transforms the photons into electrical pulses measured by a sensor circuit. Image detection in this type of detector depends upon energy of the gamma ray, statistical fluctuations in light production and quality of the Photo-Multiplier Tube (PMT). Problematically, as with gas filled detectors, scintillation detectors directly detect the effects of ionizing radiation. And, as a result, scintillation detectors must also be used within a close proximity to the nuclear materials being detected, as detected ionizing radiation quickly fades into background levels as a function of distance from the source, especially in an atmosphere environment, such as on the surface of the Earth.
Yet another type of nuclear material detector, solid state detectors, directly detect the interaction of a gamma ray within an active region of a semi-conductor. As with gas filled detectors, gamma rays generate electron hole pairs that are collected by electrodes attached to a semiconductor crystal. Solid state detectors dramatically improve resolution over scintillation detectors.
Unfortunately, like gas filled detectors and scintillation detectors, solid state detectors also only directly detect the effects of ionizing radiation and therefore must operate in close proximity to the source of the ionizing radiation, e.g., the nuclear material, if the ionizing radiation is to be detected above background levels. Thus, because all of the prior state of the art nuclear material detectors require that the x-ray or gamma ray penetrate an active volume of the detector, all of these prior detectors must be used within meters of the source of the ionizing radiation to be useful.
To the knowledge of the inventors, there heretofore has not been a detector that remotely detects (e.g., on the order of kilometers) the few gamma rays that penetrate a typical radiation shield surrounding nuclear material.
The present invention advantageously addresses the above and other needs.
The present invention advantageously addresses the needs above as well as other needs by providing an optical system for remotely detecting (e.g., from a surface platform, an aircraft up to about 20 km or perhaps even low to mid level Earth orbit satellites up to about 500 km) a selected wavelength of photon emissions from an airglow caused by a source of ionizing radiation, such as nuclear material, e.g., on Earth, by filtering enough out-of-band wavelengths (xe2x80x9crejectionxe2x80x9d) while transmitting high enough throughput for in-band wavelengths to allow detection of nuclear material with the selected wavelength(s). The transmitted wavelengths can be supported by a high enough sensor sensitivity at the selected wavelengths, and the filtering produces a low enough sensor sensitivity at out-of-band wavelengths, for a Signal-to-Noise ratio of greater than (1) one for a nuclear source generating at least 1R brightness and for a predetermined Field-of-View Structure of the optical system.
By way of example, the present invention may be used to remotely detect N2+ line emissions or other ultraviolet (UV) line emissions having defined naturally occurring properties at an operating altitude of several kilometers (instead of a few meters as required by typical gamma ray or x-ray detectors).
In particular embodiments, the increased sensitivity to detect, for example, the N2+ line emissions results from: A) a multiplication of photons; B) selection of particular spectral bands (xe2x80x9cselected wavelengthxe2x80x9d) for peak throughput of optical filters; C) an ability of photons in the selected wavelength to penetrate an atmosphere and their relative scarcity as naturally occurring photons in the atmosphere up to a certain Earth altitude (at least, e.g., 20 km); D) an ability to efficiently filter out-of-band photons with thin-film UV filters; E) the use of photon imaging devices; and F) the use of powerful signal processing algorithms.
A remote sensor for detecting a nuclear source comprises: a Field-of-View (FOV) structure having an aperture of area A and a Field-of-View (FOV) angle, the FOV angle centered on a source, and subtending a solid angle xcexa9 to the nuclear source; a plurality of optical filters for filtering photons outside a selected Ultraviolet (UV) band and for transmitting in-band photons according to a selected Transfer Function defining an out-of-band rejection ratio and an in-band transmittance ratio for the selected UV band, the Transfer Function supporting a sensor sensitivity S at the selected UV band for the Field-of-View (FOV) structure for a Signal-to-Noise ratio of greater than one (1) to detect a nuclear source of nuclear material having a brightness of at least about 1R, the selected UV band being selected such that naturally occurring in-band photons are at a brightness of about less than 104 R during daylight and are not naturally occurring at night up to about 20 km Earth altitude, and such that the in-band photons have a mean free path in air large enough and a radiative emission rate short enough to allow the in-band photons to reach the Field-of-View structure up to about 20 km Earth altitude; and an optical camera configured to receive the in-band photons transmitted through the optical filters for measuring the in-band photons, the in-band photons being emitted from airglow caused by ionizing radiation from the nuclear material.
In a variation, the remote sensor has the Signal-to-Noise ratio (SNR) computed by (S/n)It/{square root over ((S/n)It+(S/n)(cont)I(cont)t)} wherein S denotes the sensor sensitivity at the selected (UV) band, n denotes a number of pixels in the optical camera, (cont) denotes a contaminant signal, I denotes an intensity of the nuclear source at the selected UV band, and t denotes a pixel integration time.
In another variation, the remote sensor comprises a photon detector and has the sensor sensitivity S computed by:   S  =                    10        6                    4        ⁢                  xe2x80x83                ⁢        π              ⁢    A    ⁢          xe2x80x83        ⁢    Ωϵ    ⁢          xe2x80x83        ⁢    Q  
wherein: xcex5 is sensor throughput, and Q denotes a quantum efficiency of the remote sensor""s photon detector.
In a further variation, the remote sensor has the selected UV band of 3914 Angstroms.
In another embodiment, a method of remote sensing of nuclear materials, comprises the steps of: selecting an ultraviolet band such that naturally occurring in-band photons are at a brightness of about less than 104 R up to 20 km Earth altitude and are not naturally occurring at night close to the earth""s surface, and the in-band photons have a mean free path in air large enough and a radiative emission rate short enough to allow the in-photons to reach a Field-of-View structure up to about 20 km Earth altitude; defining a Transfer Function defining an out-of-band rejection ratio and an in-band transmittance ratio for the selected UV band, the Transfer Function supporting a high enough sensor sensitivity S at the selected UV band and with the Field-of-View (FOV) structure to support a Signal-to-Noise ratio of greater than one (1) for a source of the nuclear material of a brightness of at least 1R at the selected UV band; filtering out-of-band photons received through the Field-of-View structure with the optical filters having reflective properties according to the selected Transfer Function; transmitting in-band photons received through the Field-of-View structure with the optical filters having reflective properties set according to the selected Transfer Function; and counting transmitted in-band photons from the optical filters at a photon imaging device.